We developed a model for Zn metabolism in humans that measures Zn absorption, distribution, excretion and secretion. Using the model we have recently defined 3 new sites of regulation of Zn metabolism (exchange with RBC, release from muscle and secretion into gut) in addition to absorption from gut and excretion in urine. We did this with data obtained over 10 yrs with analysis by mathematical modeling. We now intend to 1) use a large unique database of 65Zn data from normal subjects to define kinetically each site of regulation and to study how regulation changes during aging, 2) to refine the model using in vitro data on RBC Zn exchange and 3) to use the model to investigate 70Zn data. 1) The 65Zn data were collected from normal subjects, aged 18-84 yrs. Subjects were given oral 65Zn and 65Zn was measured in plasma, RBC, urine, feces, whole body and over liver and thigh for 9 mo. An oral Zn load (100 mg/d) was then given for 9 mo and measurements continued. During Zn loading Zn excretion in "older" (greater than 70 yr) subjects increased 7-fold, in "younger" subjects (less than 40 yr) only 3-fold, suggesting that regulation of Zn changes during aging. The data fitted by the compartmental model showed that there were 5 sites of regulation. Parameters at each site will be determined and compared to define this change and its mechanism. 2) We collected in vitro data on 1 regulatory site, RBC exchange. Binding and uptake of Zn by whole RBC and resealed ghosts were measured in relationship to varying concentrations of Zn albumin, globulins, histidine, glycine and chelators. The model will be extended to characterize the binding and uptake of Zn to RBC and to characterize interactions of each ligand. 3) Stable isotope 70Zn data have been collected from 6 subjects given 70Zn and 65Zn orally simultaneously. Activity in plasma, RBC, urine and feces will be fitted by the model to determine similarity in metabolism of the two isotopes. Absorption, distribution, excretion and secretion of Zn will be determined from our own and other 70Zn data in the literature. The importance of each study is 1) to identify differences in the metabolism of Zn with age which may shed light upon the role of Zn in aging; 2) to understand metabolism of Zn in a cellular system (RBC) and effects of several physiological factors; and 3) to use stable isotope data not only to measure human Zn absorption, but also Zn distribution, excretion and secretion.